In a semiconductor light emitting element in which an n-type semiconductor layer, a light emitting layer and a p-type semiconductor layer are laminated on a growth substrate having optical transparency with the emission wavelength of the light emitting layer, a method for forming a reflection film having high reflectance on the side opposite to the surface, from which the light emitted from the light emitting layer is extracted, has been used as a means of improving light extraction efficiency (or external quantum efficiency). This is because the light generated in the semiconductor light emitting layer has a nature to be emitted in all directions, and as FIG. 8 shows, more lights are emitted diagonally (the output angle is large) than lights emitted upward from the emission point (the output angle is small), so most of the lights emitted from the light emitting layer are lost after performing multiple reflection inside the element.
FIG. 9 shows a change of light extraction efficiency with respect to the change of the reflectance of the reflection film. As FIG. 9 shows, 95% or higher reflectance is required to increase the light extraction efficiency to be 70% or higher, and in this 95% or higher area, light extraction efficiency improves about 6% if the reflectance improves only 1%. In the case of a GaAs semiconductor, high reflectance is implemented, and light extraction efficiency can be improved by using Au as a material of an electrode, which also functions as the reflection film.
However the reflectance of a metal greatly depends on the wavelength, and this method cannot be used on oxide or nitride compound semiconductor light emitting elements which have an emission peak in the shorter wavelength side from red light. For example, ohmic contact cannot be guaranteed between a GaN material and a high reflection metal, such as silver and aluminum. Therefore a laminated electrode of such a metal as Ni, Pt or Rh, such a metal oxide as ITO (Indium Tin Oxide), and a high reflection metal, is used, which makes it difficult to obtain a reflectance higher than the natural reflectance of the high reflection metal.
In Non-patent Document 1, a prior art to solve this problem is proposed. In this prior art, an SiO2 film with ¼ optical wavelength is layered between silver as the high reflection metal and a pGaN layer, which is a semiconductor layer, in order to guarantee a reflectance higher than the natural reflectance of the high reflection metal, and higher reflectance is obtained for all the incident angles compared with the case of silver film alone. By this, an ODR (Omni-Directional Reflector) is formed and the average reflectance becomes 98% when calculated with a 450 nm wavelength. The ohmic contact is guaranteed by means of micro-contacts in which RuO2 (ruthenium oxide) film is formed between the above mentioned pGaN layer and the SiO2 film, and the silver layer is electrically connected with the pGaN layer via the RuO2 film through the openings formed in the SiO2 film.
This prior art is supposed to implement high reflectance for all the incident angles, but as a result of the present inventors performing similar calculations, it was found out, as shown in FIG. 10, that the reflectance drops about 20% in a wide angle range centered around 55° if the film thickness of SiO2 film has a ⅛ optical wavelength film thickness (0.5Q), and the reflectance drops about 30% in an angle range centered around 45° if the SiO2 film thickness has ¼ optical wavelength film thickness (1Q). The probable reason is that if a single layer of SiO2 film is formed on metal as a reflection film, good reflection can be obtained at a ¼ optical wavelength film thickness (1Q) when the incident angle is small, but as the incident angle increases, light which effuses from the semiconductor layer to the SiO2 film as shown by the broken line in FIG. 11, in other words, light called “near field wave” or “evanescent wave” couples with the silver film layer. In FIG. 10, ¼ optical wavelength film thickness=λ/(4n)=1Q, n is a refractive index. In FIG. 10, in order to obtain the data characteristics written in Non-patent Document 1, that is 98% reflectance, using an average value of each reflectance at 0 to 90° incident angle, the thickness of the reflection film must be increased to 5Q or 6Q.
As FIG. 12 shows, the above mentioned effusing amount is zero until the incident angle θ is critical angle θc, and light effuses up to the depth of about wavelength λ at critical angle θc, then it exponentially decreases. θc=30° to 40°. Non-patent Document 1: GaInN light-emitting diodes with RuO2, OSiO2, OAg omni-directional reflector (Jong Kyu Kim, Thomas Gesmann, Hong Luo and E. Fred Schubert, Applied Physics Letters, 84, 4508 (2004), Rensselaer Polytechnic Institute)